The present invention is directed to low cost, high efficiency solar cell technology. More specifically, the present invention is related to a method and apparatus for producing photovoltaic energy using solid-state devices.
Conventional photovoltaic cells convert sunlight directly into electricity by the interaction of photons and electrons within the semiconductor material. Most solid-state photovoltaic devices rely on light energy conversion to excite charge carriers (electrons and holes) within a semiconductor material and charge separation by a semiconductor junction producing a potential energy barrier. To create a typical photovoltaic cell, a material such as silicon is doped with atoms from an element with one more or less electrons than occurs in its matching substrate (e.g., silicon). A thin layer of each material is joined to form a junction. Photons, striking the cell, transfer their energy to an excited electron hole pair that obtains potential energy. The junction promotes separation of the electrons from the holes thereby preventing recombinations thereof. Through a grid of physical connections, the electrons are collected and caused to flow as a current. Various currents and voltages can be supplied through series and parallel arrays of cells. The DC current produced depends on the electronic properties of the materials involved and the intensity of the solar radiation incident on the cell.
Conventional solar cell technologies are based largely on single crystal, polycrystalline, or amorphous silicon. The source for single crystal silicon is highly purified and sliced into wafers from single-crystal ingots or is grown as thin crystalline sheets or ribbons. Polycrystalline cells are another alternative which is inherently less efficient than single crystal solar cells, but also cheaper to produce. Gallium arsenide cells are among the most efficient solar cells available today, with many other advantages, however they are also expensive to manufacture.
In all cases of conventional solid-state photovoltaic cells, photon (light) absorption occurs in the semiconductor with both majority and minority charge carriers transported within the semiconductor; thus, both electron and hole transport must be allowed and the band-gap must be sufficiently narrow to capture a large part of the visible spectrum yet wide enough to provide a practical cell voltage. For the solar spectrum the ideal band-gap has been calculated to be approximately 1.5 eV. Conventionally, expensive material and device structures are required to achieve cells that provide both high efficiency and low recombination probability and leakage.
A conventional solid-state solar cell, such as the one shown in FIG. 1, may include structures such as a semiconductor junction, heterojunction, interface, and thin-film PV""s, which may be made from organic or inorganic materials. In all of these devices the necessary elements of these types of devices are a) photon absorption in the semiconductor bulk, b) majority and minority charge carrier transport in the semiconductor bulk, c) a semiconductor band-gap chosen for optimal absorption of the light spectrum and large photovoltages, and d) ideal efficiency limited by open circuit voltages less than the semiconductor band-gap. The photon absorption occurs within the bulk semiconductor and both majority and minority carriers are generated and transported in the bulk. For adequate absorbency, relatively thick, high quality semiconductors are needed. However, defect free, thick, narrow band-gap, materials are limited in numbers and expensive to fabricate. In heterostructures a limited number of acceptable compatible materials are available. Schottky barrier based devices have been proposed in this class that rely, again, on absorption of photons in the semiconductor bulk and use the Schottky barrier for charge separation.
Another class of conventional solar cells are the dye-sensitized photoelectrochemical solar cells as shown in FIG. 2. These devices were derived from work on photoelectrochemical electron transfer and are cathode/electrolyte/anode systems in which a photoactive molecule is light activated and oxidized (or reduced)by electron (or hole) transfer to the adjacent semiconductor electrode. The charge transfer agents, which replace the transferred charge in the photoactive molecule, are typically molecules or atoms dissolved in a liquid electrolyte such that the molecules or atoms receive charges from an electrode. Reduction is performed by an electron donor in the liquid electrolyte. This device is limited in its power output by the relative free energies of electrons in the electrolyte and the semiconductor which limit the photovoltage. The maximum photovoltage is limited by the difference between the bottom of the conduction band edge and the liquid electrolyte chemical potential. Additional inefficiencies result from the required molecular diffusion of the donors to the electrode as well as overpotential losses at the electrode/electrolyte interface.
Another solid-state solar cell is the dye-sensitized Schottky barrier solar cell as described in U.S. Pat. Nos. 4,105,470 and 4,190,950 by Dr. Skotheim. The Skotheim device is similar to the above-mentioned photoelectrochemical cell except the liquid electrolyte is replaced by a xe2x80x9creducing agentxe2x80x9d layer, the property of which is not precisely identified in either the ""470 nor the ""950 patent. Purportedly, as a means of removing the band-gap restrictions of conventional PV""s, an invention was reported by Skotheim who proposed a solid-state Schottky barrier device whereby a) photon absorption occurs in a photosensitive dye deposited on the surface of a semiconductor, b) majority charge carriers are injected directly into the conducting bands of the adjacent semiconductor, c) the ionized photosensitizer is neutralized by charges delivered by a reducing agent, d) a conductor provides charge to the reducing agent, and e) the Schottky barrier height will determine the device""s ideal efficiency and its height is determined by the interaction of the dye and the semiconductor. However, as previously mentioned, neither patent suggests the physical properties of the reducing agent, and it is unclear whether the proposed devices disclosed in the ""470 and ""950 patents can indeed yield the purported results. In the proposed cell three separate molecular oxidation/reduction electron transfer steps are required (one from the excited dye to the adjacent semiconductor, one from the reducing agent to the dye, and one from the conductor to the reducing agent). Thus an electron must move from/to a conduction band to/from a molecular orbit twice and from one molecular orbit to another one. An implementation of the device was published using an organic hole transport material, however, the performance and longevity were poor [ref: U. Bach, et al., Nature, Vol. 395, October 1998, pg. 583-585].
Experimental work by the present inventor has demonstrated that low energy molecular energy transfer at conducting surfaces can lead to excited charge carriers that can be efficiently transported through a conductor without energy loss (via ballistic transport) and captured by an electrical barrier device wherein the barrier height is determined in part by the electronic interactions between the surface conductor and the barrier material.
Accordingly, a fundamentally different type of photovoltaic device is provided by the present invention which can be easily manufactured from a wide variety of inexpensive material, and which may be, in practice, more efficient, the various embodiments of which will be described in more detail below.
The preferred embodiment of the present invention described herein is a multilayer solid-state structure wherein light absorption occurs in photosensitive layer (molecules or nanostructures) and the energetic charge carriers produced by the absorption are transported ballistically, without significant energy loss, through an ultra-thin conductor, to and over an adjacent potential energy barrier that separates and stores the charge for use as electrical power. The potential energy barrier largely determines the device efficiency and can be optimized by choice of the device materials.
In accordance with the preferred embodiment, a photoexcitable molecular species or absorbing nanostructure is deposited on an ultra-thin conductor, and following photoexcitation excited charges are ballistically transported through the conductor to the potential energy barrier (Schottky barrier) created at the interface between the conductor and the charge collection layer (a semiconductor). The ultra-thin conductor has, inter alia, three specific functions: I) allows efficient ballistic transport of charge carriers from the photosensitizer to the potential barrier at the interface, II) directly provides replacement charges of the opposite sign to the ionized photosensitizer, and III) influenced, in part, by its interaction with the charge separation layer, the magnitude of the potential energy barrier which determines, in part, the maximum device power.
The essential components (e.g., layers) of the preferred embodiment of the present invention include: 1) a photosensitive layer where light energy is converted to electron and/or hole excitation, 2) an adjacent ultra-thin conducting layer that provides a pathway for ballistic transport of charges using high efficiency conduction bands, and as a source of replacement charges to the photosensitive layer; and 3) a charge separation and collection layer such as an inorganic or organic semiconductor affixed with a back side ohmic contact opposite the ultra-thin conducting layer. The ohmic contact collects the charges transported across the barrier. The addition of an anti-reflection coating on top of the device is a highly practical embodiment of the invention.
The present invention is advantageous over the aforementioned dye-sensitized Schottky barrier solar cell structure in that it has the advantage of potentially greater photovoltages due to the ability to influence the barrier height by the choice of a high (for n-type semiconductors) or low (for p-type semiconductors) work function conductors at the surface, by the choice of the semiconductor (type and doping level), and by the surface treatment of the semiconductor prior to disposition of the conductor to maximize the barrier height by affecting the interface. Additional advantages of the present invention include eliminating the need for a specific reducing agent or a minority charge carrier transport material, and providing the ability to choose from among a broad choice of charge separation layer material to include both wide band-gap n and p type semiconductors. In contrast to the prior art, U.S. Pat. Nos. 4,105,470 and 4,190,950 by Skotheim, only two transfers of electrons to/from conduction bands are required and no intermolecular charge transfer is necessarily required.
It is an object of the present invention to: 1) eliminate the need for electrolytes and/or molecular reducing agents and/or minority carrier conductors, 2) allow for a wider choice of the conductor and charge separation layer, and 3) maximize by design of the open circuit photovoltage.
It is another object of the present invention to increase the efficiency of photovoltaic energy generation. More specifically, light absorption can be optimized since a single band-gap is not required for light absorption, and a large number and variety of materials with selectable spectral properties of photoabsorbing molecules or structures can be utilized without the need for compatibility with an electrolyte. Without the overpotential losses of the electrochemical redox reactions (both at the electrodes in the photoelectrochemical cell and by the reducing agent charge transfer), higher efficiencies are also possible. The ultra-thin conductor is used as an efficient ballistic transport channel and to maximize the photovoltage as determined by its effect on the barrier. The interaction between the conductor and charge separation layer to influence the barrier height, can be optimized by the choice of the conductor, charge separation material, and interface preparation.
It is yet another object of the present invention to lower the cost of generating photovoltaic energy. More specifically, present solid-state P.V. systems are expensive due to the need for high purity low defect silicon or other semiconductors with the required band-gap, which have high manufacturing costs. The liquid containing photoelectrochemical cells have reliability and efficiency limits as well as restrictions on the dye stability and reducing agent in solution, thus increasing their in-use costs. Frequently, reactive species such as iodine must be used.
In is yet another object of the present invention to increase the longevity of the solar cell devices by using stable components. In the case of photoelectrochemical cells, most types of feasible electrolytes are reactive and can erode or dissolve the adjacent semiconductor or react with the dye, causing the device to be unstable. By eliminating the need to use reactive components, the present invention promotes the longevity of solar cells.